OSLO, Norway (AP) - Norwegian glaciers shrank for the fourth straight year in 2004 because of warm summers and winters with little snow. Some receded at the fastest rate since measurements started in 1900, a report issued Tuesday found.

Glaciers build up year after year from snow that eventually turns to ice if it does not melt during the summer, and can have foundation ice that is 5,000 years old.

They have been shrinking worldwide over the last century because of global warming, according to the Norwegian Water Resources and Energy Directorate.

However, Bjarne Kjoellmoen, the chief engineer of the directorate, was reluctant to predict broad trends for Norway based on four years of measurements, since many of the same glaciers had grown in the previous 15 years.

"It is also a little difficult to predict a trend in a country this long," he said, referring to the fact that Norway is nearly as long as continental Europe.

The report said the 22 of the 26 glaciers measured last year had decreased in size, while one in northern Norway had grown.

The biggest glacier on the European mainland, Jostedalsbreen in western Norway's Sogn og Fjordane province, has been among those shrinking, according to the directorate's report.

It said an arm of that glacier had receded by what amounts to a one-meter layer of water over its width during 2004.

Another arm of Jostedalbreen, called Briksdalsbreen, receded nearly 100 metres in length in one year, the largest annual retreat for that glacier since measurements were first taken in 1900.

Kjoellmoen said natural cycles and greenhouse gas driven by global warming may both be part of the explanation.

He said many glaciers were stable from the 1930s through the 1960s, with the amount of winter snow equalling the amount of summer runoff.

"Then there was about 15 years of growth," Kjoellmoen said. That period lasted until about 2000 on most of the glaciers, although there are strong regional differences.

"The glaciers have been shrinking for about 100 years," he said, adding that some areas, such as the western coast, haven't had enough snow to maintain the glaciers for a century.

The directorate closely monitors Norwegian water levels, including projected glacial runoff, because virtually all of Norway's electrical power is hydroelectric even though it is the world's third-largest oil exporter.